Sputtering, alternatively called physical vapor deposition (PVD), has long been used for the deposition of metals such as aluminum to form horizontal interconnects in a semiconductor integrated circuit. More recently it has been adapted to deposit thin liner layers in high aspect-ratio via holes used to form vertical interconnects between metallization layers in the integrated circuit. Examples of liner layers include barrier layers such as of titanium and tantalum and their nitrides and copper seed layers subsequently used in filling the via holes with copper by electrochemical plating (ECP).
DC magnetron sputtering is the preferred sputtering method in commercial fabrication. It has been adapted to sputtering into high aspect-ratio via holes by increasing the fraction of sputter metal atoms that are ionized and by electrically biasing the wafer so that the ions are accelerated towards the wafer at high energy and penetrate deep within the via hole.
Even more recently, it has been found that a plasma sputter chamber can be used to sputter etch the wafer, also called resputtering, so that the same sputter chamber can be used in a multi-step process to both sputter deposit material onto the wafer and to remove material from the wafer. The geometry associated with high aspect-ratio via holes allows the two steps to be controlled such that on net material is deposited on only some portions of the via hole. This selective deposition is particularly useful in depositing a metal nitride barrier layer, which is somewhat resistive, onto the sidewalls of the via hole but not depositing it on the bottom of the via hole so that the underlying lower-level metallization can be directly contacted by the vertical metallization.
A conventional DC magnetron sputter chamber includes a metallic target, for example, having at least a surface layer composed of tantalum or copper, which is selectively negatively biased with respect to the chamber sidewalls or protective sidewall shields so that a sputter working gas of argon is discharged into a plasma. The magnetron placed in back of the target projects a magnetic field to the other face of the target to concentrate the plasma adjacent the sputtering surface of the target. The argon ions are energetically attracted to the target and sputter metal atoms from it, some which strike the opposed wafer and deposit a metal layer on it. Such a sputter chamber can be adapted to perform sputter etching of the wafer, for example, by including an RF coil around the space between the target and the wafer to inductively coupled RF power into the chamber. In the sputter deposition phase, the RF coil is typically not significantly powered. However, in the sputter etch phase, the RF coil is powered while the DC power applied to the target is significantly reduced. The RF coil inductively couples RF energy into the chamber to excite argon into a plasma in the space above the wafer. The electrical bias applied to the wafer, which is typically a negative DC bias induced by another RF source, energetically attracts the argon ions to the wafer so that they preferentially etch horizontal surfaces, in particular the bottom of the via holes, while not significantly etching the via sidewalls.
A typical configuration of the RF coil system is schematically illustrated in FIG. 1. An RF coil 10 is positioned inside the sidewall sputter shields coaxial with the chamber's central axis 12. For economic and operational reasons, it is typically formed as a single-turn coil in a plane perpendicular to the central axis 12 wrapped generally symmetrically about the central axis 12 but having a junction 14 with two electrical leads to an external biasing circuit. The junction 14 creates an azimuthal gap in the RF coil of less than 15° although the location of the electrical contacts increase the effective gap to about 18°. Two coupling capacitors 16, 18 electrically couple the RF coil 10 between an RF power source 20 and ground. Additionally, a matching circuit 22 matches the 50 ohm coaxial cable from the RF source 20 to the impedance of a plasma 24 excited from the argon gas inside the coil 10. The argon ions from the plasma 24 are attracted towards the negatively biased wafer to sputter etch it.
It has been found that the plasma 24 tends to diffuse to the grounded shields or chamber sidewalls so that the radial distribution of argon ion striking the wafer is stronger at the wafer center than at the wafer edge. Accordingly, it is known to position a DC electromagnetic coil outside the chamber and coaxial with the central axis 12 to create a magnetic barrier to the diffusing plasma, specifically the plasma electrons but the plasma ions substantially redistribute themselves to neutralize the plasma ions. The charged plasma particles interact with the magnetic field B according to the Lorentz forceF=qv×B, where q is the charge of the plasma particle, v is the particle's vector velocity, and F is the Lorentz force exerted on the charged particle by the magnetic field B. The Lorentz force F deflects the charged particle transversely to its velocity v. The radius of curvature r of the deflection is related as
                    mv        2            r        =          qv      ×      B        ,where m is the mass of the charged particle. In the limit of a charged particle trapped in a uniform magnetic field, the radius r becomes the cyclotron radius. A magnetic field B extending parallel to the central axis near the sidewalls of the chamber deflects the charged particles, particularly the lighter electrons, diffusing outward from the central axis back towards the central axis. To maintain charge neutrality inside the plasma, the charged ions follow the electrons and thus are also effectively deflected back towards the center. Gung et al. discloses a more complicated magnet array in U.S. patent application Ser. No. 11/119,350, filed Apr. 29, 2005, incorporated herein by reference and published as U.S. patent application publication 2005/0263390. Their magnet array includes two pairs of coaxial electromagnet coils, in which each pair includes coils at a same height but different radii from the chamber center.
While these electromagnet arrays have been effective at producing a more radially uniform sputter etch, further investigation has revealed that the etch rate is also azimuthally non-uniform and the magnitude of the azimuthal asymmetry nears that of the reduced radial asymmetry enabled by the electromagnet coils. That is, despite the apparent geometric symmetry about the central axis including the time-averaged rotated magnetron, the etch rate has been observed to significantly vary in the azimuthal or circumferential direction. It is greatly desired to reduce the azimuthal non-uniformity for the increasingly more stringent uniformity requirements imposed by advanced integrated circuits.